Solutions capable of plating metals onto substrates are widely used in industry. The most commonly used metal plating solutions include electrolytic and electroless solutions.
Electroless nickel plating solutions, in particular, have come into widespread usage in the manufacture of computer memory discs. Such solutions generally contain a nickel metal salt, such as the sulfate, acetate, carbonate or chloride salt, for the source of dissolved metal plating ions, a reducing agent, such as sodium hydrosulfite, sodium hypophosphite, sodium borohydride, boranes or hyrdazines, to reduce the metal ions to metallic form, a complexing or chelating agent, such as monocarboxylic acids, dicarboxylic acids, hydroxycarboxylic acids, amino acids, and alkanolamines, to maintain the metal ions in solution and prevent premature precipitation, and a pH adjuster to maintain the solution pH typically within an acidic range of 4-5. Accelerators, stabilizers, buffers, and wetting agents may also be included in the electroless nickel solutions.
These electroless nickel solutions only have a limited useful life and eventually become depleted or spent. Typically, after a limited number of plating cycles, the concentration is reduced of both the complexed metal in solution (by plate-out onto the substrate) and the reducing agent (by consumption according to the chemical reaction controlling plating) to where the plating rate of the electroless bath is slowed sufficiently to become unsatisfactory, and the bath has to be discarded.
Disposal of such spent electroless nickel solutions, however, presents a major problem, since a large percentage of the original complexed nickel content remains dissolved in the spent solutions. From an environmental standpoint, it is well known that heavy metal ions and particularly those of the metal here of interest, namely nickel, can have adverse toxic effects on the environment if directly discharged in soluble form into effluent wastewater streams that feed into municipal water systems or natural bodies of water. Also, such highly chelated streams generally cannot be mixed with unchelated streams at the wastewater treatment plant, making processing much more difficult. As a result, discharging dissolved heavy metals, including nickel, directly into effluent wastewater streams, in other than minute quantities, is prohibited by local, state, and federal regulations.
A number of wastewater treatment processes have been proposed to reduce the metal content in spent electroless solutions to low levels prior to discharge. Previously, many users of electroless baths simply dosed their wastes with caustic soda to precipitate the bulk of the heavy metal contaminant as insoluble hydrous oxides (metal hydroxides), whereupon the hydrous oxide sludge was pressed into a filter cake, drummed, and disposed. This method of metal removal, however, was known to produce a very large quantity of metal-bearing sludge, all of which needed to be disposed of in approved hazardous landfills. Sludge disposal is very expensive, environmentally detrimental, wasteful of natural resources, and involves compliance with local, state and federal regulations. In addition, the sludge producer assumes perpetual liability for the sludge deposited in the landfill site which is undesirable. Another problem with this treatment method is that it is inefficient and generally did not work well due to the high chelant levels in the spent solutions.
Another waste treatment method previously used for spent electroless solutions was to simply electrolessly plate out the metal by dosing the solution at slightly alkaline pH with reducing agents. The reducing agents typically used to convert the dissolved metal salt into insoluble metal precipitate include sodium borohydride, sodium hydrosulfite, sodium hypophosphite, boranes and hydrazines. An advantage of this method is that the metal-bearing sludge produced contains high levels of elemental metal which can be reclaimed by smelting and sold at a profit. Therefore, hazardous waste disposal of the sludge is no longer required. Plainly, this method is economical, environmentally friendly, and conserves natural resources. One drawback, however, is that the soluble nickel content in the treated electroless nickel baths can only be partially reduced. Generally, the nickel content can only be lowered to about 3-70 parts per million (ppm) which no longer meets the discharge limits in most jurisdictions.
Still another prior waste treatment method known for reducing the dissolved metal content of spent electroless baths to acceptable discharge levels involves organosulfur precipitation of the metal by dosing the spent solution at a pH of 5-8 with water-soluble sodium dithiocarbamate (DTC) precipitating agents, such as sodium dimethyldithiocarbamate (DTC-Na). The dissolved metal salt complexes with the soluble dithiocarbamate salts to form insoluble metal dithiocarbamate precipitates. The metal dithiocarbamate particles are generally very fine and not conducive to settling, and typically require the aid of coagulants and flocculants to form larger, faster settling flocs which are more capable of removal by filtration. The dithiocarbamate method effectively removes the heavy metal content in spent electroless baths to non-detectable levels, but undesirably produces huge quantities of heavy metal-bearing carbamate sludge which create potentially dangerous hydrolytic loading on the typical liquid-solid system. Furthermore, these sludges must be classified as hazardous waste and disposed of predominantly in approved hazardous landfills. Here again, sludge disposal is very expensive, environmentally detrimental, and wasteful of natural resources.
Prior attempts to waste treat spent electroless nickel solutions with borohydride reduction followed by dithiocarbamate precipitation without removing the reduction precipitants or lowering the pH prior to dithiocarbamate precipitation have generally been met without much success. With the combined method, no appreciable differences in the nickel levels have been shown over straight borohydride reduction, which levels fall above current discharge limits.
While the prior processes reduce the metal content of the spent electroless plating solutions, the need still exists for efficient and economical methods for waste treating such solutions which also meet current discharge limits in the low parts per million without simultaneously generating significant amounts of hazardous waste. Moreover, as the regulatory rules for waste stream effluent discharge limits become more stringent, such as to certain fractional parts per million, a search for improved processes to remove the metal content becomes mandatory.